As spherical particles exhibiting different properties by hemispheres, fine particles individually having a black hemisphere surface and a white hemisphere surface, i.e., black/white bichromal balls, for example, have been used in displays. Further, composite fine particles individually having a hydrophilic hemisphere surface and a hydrophobic hemisphere surface, have been used as surfactant particles for dispersion stabilizers, and may also be used as components of composite fine particles.
As a process for production of such composite fine particles with heterogeneous hemisphere surfaces, there have been known a microreactor method and a nozzle method. These methods either join two kinds of liquid droplets respectively produced from different nozzle ends, or join such two kinds of liquid droplets in a small channel in a microreactor. Problems imposed by the aforementioned methods include: (i) significantly low productivity; (ii) difficulty in realizing a stable operation due to complexities of interface and flow conditions for joining two liquid droplets as one; (iii) limitation in a controllable range of particle diameter due to the fact that the liquid droplet(s) are produced through nozzles or small channels; (iv) restrictions on heterogeneous substances added for functional expression, especially on solid powders due to the fact that the droplet(s) are produced through nozzles or small channels and that a solid powder may thus block the nozzles or small channels; and (v) inability to encapsulate a heterogeneous substance in central portions of composite fine particles.